1. Field of the Invention
The invention relates generally to a method of fabricating a semiconductor device, and more particularly, to a method of fabricating a small contact window having a large aspect ratio.
2. Description of the Related Art
As the level of integration of semiconductor devices increases, it becomes inevitable that the design rule of semiconductor devices is reduced. However, the reduction of the design rule of a device is not made at an identical or exact ratio with respect to all dimensions of the device. For example, the thicknesses of an interlayer insulative layer or a wiring layer involve vertical dimensions that cannot be reduced exactly in proportion to a change in design rule because other factors determining the thickness of each layer must be considered, e.g., breakdown voltage, parasitic capacity, current capacity, wiring resistance, etc. Thus, an aspect ratio is a ratio of the depth of the contact window to the width of the lowest portion of a small contact window.
When the aspect ratio is increased, either a contact window does not become completely formed or an incline phenomenon occurs when etching to form the contact window wherein the width of a bottom portion of a contact window becomes narrower than that of an upper portion of the contact window. When an incline phenomenon occurs, a contact area between the contact window and a lower conductive film is decreased, thereby precipitously increasing sheet resistance.
Particularly, in a capacitor over bit line (COB) structure where a capacitor is formed after forming a bit line, because the aspect ratio of a contact window for contacting a lower electrode of a capacitor with an active region formed on a semiconductor substrate is so large, and the contact area at the lowest portion of the contact window becomes significantly smaller, the sheet resistance is greatly increased.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.